Radioimmunoassays of prostaglandins in homogenates of human endometrium indicate that the endogenous concentrations of E and F types are nearly equal and do not vary significantly throughout the menstrual cycle. Thus, the observation that uterine levels of prostaglandins are higher during the secretory phase than during the proliferative phase of the human menstrual cycle is due to the increase in endometrial tissue rather than to a change in tissue concentrations of prostaglandins. The radioimmunoassay results pertaining to F1 alpha and F2 alpha are being confirmed by an electron-capture gas chromatographic analytical procedure previously developed in this laboratory. This analytical procedure, which involves purification by thin-layer chromatography and conversion to triheptafluorobutyrate methyl ester derivatives, is being expanded to include the analysis of F3 alpha. A similar procedure for measuring sub-microgram amounts of E prostaglandins is also under investigation.